In the circuit below, the texts mention the problem with this kind of biasing here is if the hfe(β) varies then the Ic will vary in linear reagion. (What varies what here also not clear, I thought all variation source was Vbe). Anyways:
Their argument is the following:
Imagine for this circuit we want to size R4 for a desired Ic, known R3, known supply voltage Vcc and known β.
So we can say the required Ib is:
Ib = Ic/β
and we can write a KVL equation to find R4:
Vcc-Vbe = Ib x R4
R4 = (Vcc -Vbe) / Ib
Then they say this circuit is biased well but if the β varies i.e if you rely on datasheet, or change the transistor ect, the circuit will fail.
I can understand that, but my problem is when they reach sizing R4 in their calculation they take Vbe as a fixed value like 0.7V. But indeed Vbe changes with R4 and we also know that a small change in Vbe has a huge impact on Ic.
I don't know why they don't mention this as a problem. I'm wondering if I misunderstand something here. So my question is, imagine if the β were guaranteed to be fixed as a constant and we don't have a problem with β variation. Don't we still have a problem here not knowing Vbe's exact value when sizing R4 from that KVL equation? I mean Vbe varies with R4 but they still take it as 0.7V.
(Shouldn't they find the real Vbe form Eber Molls equation to find the accurate R4? But for that we need reverse sat current.)